1. Field of the Invention
The invention relates to a process for production of directly reduced iron in a multi-stage furnace.
2. Discussion of the Background
Production of directly reduced iron takes place in a direct reduction process by reduction of iron oxide with solid or gaseous reducing agents. A carbon carrier, which reacts with carbon dioxide and forms the reduction gas CO at higher temperatures, for example, serves as a solid reducing agent.
A process of this kind can be carried out, for example, in a rotary hearth furnace, i.e. in a furnace with a rotatable annular furnace bottom, which is lined with refractory material on the top side and is enclosed by a casing. Burners, which penetrate the casing and heat the interior of the casing to the required reaction temperature of over 1000xc2x0 C., are mounted on the top of the casing.
The iron oxide is spread together with the reducing agent at a specific point on the rotary hearth and is introduced by the rotation of the rotary hearth into the interior of the casing, where it reacts with the reducing agent because of the high temperatures and is present as directly reduced iron after about one revolution of the rotary hearth. In this process iron oxide and reducing agent after charging on to the refractory lining of the rotary hearth must first be heated to the required reaction temperature before the actual reduction reaction can begin. This takes place in the area bordering on the charging zone of the rotary furnace in the direction of rotation by heat transfer from the hot waste gases of the burners to the charged materials.
Because of the low thermal conductivity of the charged materials, the heating-up phase takes a considerable time before the required reaction temperature is achieved within the charged material layers. The longer the heating-up phase, the lower is the productivity of the rotary hearth furnace, because the heating rate determines the speed of rotation of the rotary hearth.
The reduction process depends on the concentration of the reduction gases, which are in contact with the ore. However, the gas composition in the individual furnace zones can hardly be affected, because the entire furnace consists only of a single process space. In the conventional processes the diffusion of the CO from the reducing agent to the ore and CO2 from the ore to the reducing agent thus cannot be affected.
From a certain degree of metallisation onwards the speed of the reduction process diminishes in such a way that the process is usually interrupted when a degree of metallisation of 85-95% is achieved. Uneconomical extension of the process time would be required to reduce the remaining oxides.
The document DE 1 225 673 relates to a process for dry reduction of iron ore in a multi-stage furnace, which has several stages one above the other. The iron ore is charged to the top stage and gradually transferred to the lower stages. In the lower stages (reduction stages) a reducing gas is fed in order to reduce the iron ore. In the upper stages the iron ore is preheated to the required reduction temperature by combustion of the rising reducing gas. Before introduction into the multi-stage furnace a solid reducing agent can be mixed with the iron ore. Part of the reducing gas from at least one of the upper reduction stages is removed and fed into at least one of the lower reduction stages.
A process for production of sponge iron in a multi-stage furnace, which has several stages one above the other, is already known from document U.S. Pat. No. 2,089,782. The iron ore is charged to the top stage and gradually transferred to the lower stages. Solid reducing agent is charged to one of the stages underneath. The iron ore is reduced in the lower stages (reduction stages). The thermal energy required for the reduction is supplied by an electrically heated melt provided under the bottom stage of the multi-stage furnace. In the upper stages the iron ore is preheated by combustion of the reducing gas rising from the reduction stages.
Consequently the task of the present invention is to propose an alternative process for production of directly reduced iron.
According to the invention this problem is solved by a process for production of directly reduced iron in a multi-stage furnace which has several stages one above the other, a high temperature prevailing in the lower stages and in which
ore is continuously introduced into the multi-stage furnace and deposited on the top stage and gradually transferred to the lower stages;
reducing agent is deposited on the topmost stage and/or on one of the stages underneath it;
a gas containing oxygen is fed into the lower stages and reacts with the reducing agent to form reducing gas, the reducing gas reacting with the ore to form directly reduced iron;
the directly reduced iron is discharged together with residues of reducing agents in the area of the bottom stage of the multi-stage furnace.
An important advantage of the invention is that the process space is subdivided into different zones, the solids move continuously from the top downwards and the gases from the bottom upwards. By subdividing the process space into different zones the process conditions can be measured and controlled in the different zones or even for each stage and selectively .
Solid, liquid or gaseous reducing agents come into consideration as reducing agents.
In this process fine-grained ore can be charged and caking avoided by selective process control and continuous circulation. This is particularly advantageous, if ash-forming reducing agents are used. The separation of the ash of the reducing agent from the iron can be easily carried out. This separation can take place, for example, in the hot stage by screening. After partial cooling below 700xc2x0 C. it is possible on the other hand to separate the directly reduced iron via magnetic separators from the ash and excess reducing agent. Hence this process can be used, because the continuous agitation in the multi-stage furnace prevents caking of the iron. The directly reduced iron is accordingly produced in fine-grained form and is easily picked up by the magnetic separators. The quality of the directly reduced iron obtained in this way is independent of the quantity of residues of the reducing agent.
The iron obtained can subsequently be processed into pellets or briquettes or introduced directly into a melting furnace (electric furnace etc.) and further processed.
If required, the reducing agent residues produced are burnt in burners with any unused reducing agents and the resulting heat fed to the furnace.
Accordingly a less expensive reducing agent which has a relatively high ash content can be used and/or work carried out with a relatively high excess of reducing agent.
In cases in which it is necessary to work with an excess of reducing agents, it is advantageous to treat the residues in order to separate the unused reducing agents and reuse them. This can be done e.g. by screening the residues, if the unused reducing agents are present in a sufficiently coarse form. The unused reducing agents can be introduced directly into the multi-stage furnace.
However, the charge of reducing agents can also be divided among several stages.
It is thus possible for coarse-grained reducing agents (1-3 mm) to be introduced at a higher point into the multi-stage furnace and fine-grained reducing agents ( less than 1 mm) added at a lower point. Consequently discharge of dust with the exhaust gases is largely avoided and the reaction accelerated by the fine reducing agent particles introduced lower down.
The charging of coarser particles reduces the consumption of reducing agents, because the small particles are consumed faster via waste gases in the upper stages than is necessary for reduction of the iron ore.
According to a preferred embodiment the ore is dried and possibly preheated by the hot gases in the multi-stage furnace before it is fed into the multi-stage furnace and comes into contact with the reducing agent. The ore is preferably heated to a temperature of at least 200xc2x0 C., preferably to at least 350xc2x0 C. In this case the heating and drying time should not exceed 10 to 20 minutes in order to avoid sticking of the ore in a reducing atmosphere.
The ore can however be mixed with at least part of the required reducing agents before it is charged into the multi-stage furnace.
By selective addition of reducing agents in the lower stages of the furnace the reducing gases in the furnace can be adjusted to an optimum concentration, thus achieving a better degree of metallisation.
All the rising gases, including the volatile components of the reducing agents, can be completely burnt in the upper part of the furnace or outside the multi-stage furnace in the drying plant for the ore and, if appropriate, for the reducing agents, and the residual heat of the furnace""s waste gases can in this way be used to maximum advantage.
The ore is continuously circulated by rakes mounted on each stage of the furnace and gradually conveyed to the underlying stage. In this way the ore is dried and heated more quickly than in conventional furnaces. The reducing agent is quickly mixed under the ore by the rakes and quickly heated to reaction temperature. Caking of the reducing agent and ore is prevented by the continuous circulation. The rate of circulation depends on many factors such as the geometry of the rakes, thickness of the layers, etc. The ore, any reducing agent present and the directly reduced iron at the stages should be circulated at least once every one to three minutes, with the result that agglomeration is largely prevented.
It is possible to inject gases containing oxygen selectively on the stage where the heat requirement must be covered by combustion of the excess process gases.
It is advantageous to use gases containing oxygen which have a temperature of at least 350xc2x0 C.
A gaseous reducing agent can additionally be injected at the bottom stages of the multi-stage furnace. Consequently more complete reduction of the ore is achieved.
According to a further advantageous embodiment one or more stages in the furnace which are below the stage to which the reducing agents are introduced are heated by burners.
In order not to reduce the concentration of reducing gases in the lower part of the furnace by flue gases of the firing system, energy can also be supplied indirectly, i.e. by radiation heating.
According to another preferred embodiment gases are exhausted from the multi-stage furnace at one or more stages. These hot gases can subsequently be passed either through a CO2 scrubber to reduce the gas quantity and increase the reduction potential of the gas or through an additional reactor containing carbon, so that the carbon dioxide present in the hot gases reacts with the carbon to form carbon monoxide according to the producer-gas equilibrium and the reduction potential of the gas is thus increased. The gases enriched by carbon monoxide are subsequently returned to the multi-stage furnace.
If necessary, additives are introduced to one of the stages under the stage where the reducing agents are introduced.
In such a case it is advantageous to exhaust gases at a stage above the stage at which additives are introduced.
According to a preferred embodiment gases are exhausted from the multi-stage furnace below a specific stage and subsequently re-injected into the furnace above this stage. Iron oxide dust containing carbon and heavy metal can be introduced into the furnace at this stage. The heavy metal oxides are reduced there, the heavy metals volatilise and the gases produced at this stage are then separately exhausted.
To achieve a further increase in productivity the multi-stage furnace can be operated at a specific excess pressure. In contrast to a rotary furnace, which is sealed by water seals with a diameter of about 50 m, this can be realised very easily in a multi-stage furnace, which has only small seals on the drive shaft. In such a case pressure locks for the feed and removal of material must be provided.